This invention relates to the recovery of copper. Copper recovery from copper sulphide concentrates is usually accomplished by pyrometallurgical processes. Copper concentrates which may be recovered by froth flotation are dried and smelted in many types of furnaces to a copper matte which is further refined to copper metal.
Hydrometallurgical treatment methods have been developed and many processes have been reported. Very few have, to the applicant's knowledge, been taken to commercial reality.
Copper is often recovered by solvent extraction and electrowinning, but this process is normally applied to oxide ores. Sulphide copper ores of low grade are treated by dump leaching and heap leaching. The solutions generated in this way are treated by solvent extraction and electrowinning to produce pure copper metal at the cathode. Bacterial action may also be responsible for the dissolution of copper.
Copper concentrates with acid soluble copper are sometimes treated by leaching in agitated tanks with a suitable acid such as sulphuric acid. The solutions can either be processed directly by electrowinning or can be processed through solvent extraction before electrowinning.
Many copper concentrates are low in grade and this makes smelting expensive. Some copper concentrates contain elements such as arsenic which make their processing by smelting objectionable. Smelting companies are reluctant to purchase concentrates, and apply penalties according to the content and nature of such elements.
Dissolution of copper concentrates is known. Copper minerals such as chalcocite, bornite, covellite, digenite, enargite and tetrahedrite will dissolve in ferric sulphate. Other leaching agents such as ferric chloride and ammonia are used, but there are drawbacks relating to their corrosive nature or their cost.
Ferric sulphate is a leaching reagent which can be used for the copper minerals mentioned, but its disadvantage is that the reagent is consumed and converts to ferrous sulphate. The solution must either be very concentrated in ferric sulphate or the amount of copper concentrate treated must be very small in relation to the solution volume. This means a large ratio of solution to solids.
Bacterial action is known to be able to convert ferrous sulphate to ferric sulphate. Advantage can be taken of this phenomenon to regenerate ferric sulphate during leaching. Thus a smaller volume of solution can be used, because the ferric sulphate leaching agent is repeatedly used to leach more concentrate. As an example, the leaching of the mineral chalcocite takes place in the following manner: EQU CU.sub.2 S+2FE.sub.2 (SO.sub.4).sub.3 .fwdarw.2CuSO.sub.4 +4FeSO.sub.4 +S
Ferrous sulphate can be regenerated by bacterial action. Air is blown into an agitated tank where leaching and regeneration take place simultaneously in accordance with the following equation: EQU 4FeSO.sub.4 +2H.sub.2 SO.sub.4 +O.sub.2 .fwdarw.2Fe.sub.2 (SO.sub.4).sub.3 +2H.sub.2 O
There is a disadvantage in this procedure for sulphuric acid must be added to the agitated tank to satisfy the second reaction. Some sulphuric acid is generated by bacterial action whereby sulphur which is formed in the first reaction is oxidised to sulphuric acid, but it is insufficient to satisfy the oxidation reaction: EQU S+.sup.3 /.sub.2 O.sub.2 +H.sub.2).fwdarw.H.sub.2 SO.sub.4
If the three reactions are added together it is found that sulphuric acid must still be added to the agitated reactor: EQU Cu.sub.2 S+H.sub.2 SO.sub.4 +.sup.5 /.sub.2 O.sub.2 .fwdarw.2CuSO.sub.4 +H.sub.2 O